The invention relates to a linear thermal evaporator for vacuum vapor depositing apparatus, especially for continuous web coating apparatus, having an elongated crucible of a heat-resistant, nonmetallic material to contain the material to be evaporated. The crucible has an elongated cavity and, on its upper side, a slot for the emission of the vapor, which is defined on its longitudinal edges by two plates of a likewise heat-resistant, nonmetallic material. The crucible has in its cavity a heating rod parallel to the slot, which is of the same length as the slot and is disposed beneath the slot. The crucible is set within a thermal barrier system.
An evaporator of this kind is disclosed by U.S. Pat. No. 2,440,135. In this case, however, only a single heating rod is disposed beneath the center of the vapor emission slot, and not only has to supply the heat for evaporation, but also to compensate most of the heat losses, which at high evaporation temperatures are unavoidable in spite of the thermal barrier system. Consequently, the heating rod must be heated for the production of a sufficiently high temperature, which is considerably greater than the evaporating temperature needed, so that a great temperature gradient occurs between the heating rod and the exterior of the evaporator crucible. This is not helped by the fact that each of the plates defining the vapor emission slot has its own resistance heater whose only purpose is to prevent the vapor from condensing at the mouth of the evaporator.
In the operation of such an evaporator for the purpose of the thermal evaporation of high-melting glasses and/or ceramic substances such as silicon monoxide and silicon dioxide, it has been observed that at least a portion of these substances gives up oxygen to the vacuum produced by the process conditions, and the oxygen reacts with the materials of the crucible or of the crucible lining and of the plates forming the vapor emission slot. Pieces or moldings of graphite are used as a rule as the heat-resistant, nonmetallic materials for the stated purpose, and it has been found that this graphite reacts with the oxygen to form carbon monoxide. Aside from the fact that some of the oxygen is undesirably removed from the material being evaporated, so that under-stoichiometric compounds are formed, and aside from the fact that this oxidation of the materials leads to premature failure of the evaporator, the equilibrium between the internal pressure in the vacuum chamber and the pumping power of the vacuum pumps is considerably upset, so that a rise in pressure undesirably occurs in the vacuum chamber. If it were desired to counter this effect with the power of the vacuum pumps, a considerably greater amount of power would be required, and therefore greater cost of operation of the vacuum depositing apparatus. Furthermore, the heater situated centrally beneath the vapor emission slot, which is therefore necessarily in line of sight with the substrate, has the unpleasant effect of greatly raising the temperature of the substrate. Consequently, not only is additional thermal energy lost to the vapor depositing process, but also the substrate has to be vigorously cooled, especially when delicate materials are involved, such as thermoplastic films. For great radiation intensities and great film thicknesses, such cooling, however, is mostly ineffectual, because such film materials are very poor thermal conductors, as a rule.